Space cooling systems, including both refrigeration and comfort cooling systems, typically include one or more evaporators in heat exchange relationship with a space to be cooled, a condenser external to the space, a compressor for circulating a heat transfer medium, such as a vapor compression refrigerant, between the evaporator and the condenser, and an expansion valve located between the condenser outlet and the inlet to each evaporator. Each expansion valve may be positionable at various intermediate positions between a fully open position and a fully closed position to regulate the flow rate of the heat transfer medium through the evaporator. An indoor fan is usually provided to direct a flow of cooling air across the evaporator and an outdoor fan is usually provided for cooling the condenser.
Modern-day space cooling systems may also include a microcomputer programmed to control operation of the system based on inputs from various temperature and pressure sensors. Each expansion valve may be controlled in response to the measured temperature differential across the corresponding evaporator. This temperature differential is commonly referred to as the evaporator superheat. Various techniques for controlling the expansion valve in response to evaporator superheat are set forth in U.S. Pat. Nos. 4,067,203; 4,523,435; 4,617,804; 4,620,424; 4,674,292; 4,787,213; and 5,551,248.
Space cooling systems also typically include some type of mechanism which is operable to prevent frost build-up on the evaporator(s) and a device for controlling operation of the defrost mechanism. Defrosting may be accomplished in a number of different ways, including using an electrically resistive heating element to heat each evaporator, introducing hot gas into the evaporator, or operating the indoor fan to melt the frost accumulated on the evaporator. The defrost operation may be initiated based on a pre-programmed time between successive defrost operations, or, alternatively, in response to selected indicators, such as evaporator temperature, ambient air temperature, optical detection of frost build-up, compressor run time, or evaporator cooling fan performance. The defrost operation may be terminated in response to a predetermined elapsed time since the onset thereof, or, alternatively, in response to an indication that the evaporator temperature has reached a predetermined target temperature. Prior art examples of defrost controls therefor are shown in U.S. Pat. Nos. 4,338,790; 4,406,133; 4,573,326; 4,882,908; 5,315,835; and 5,415,005; and in European Patent Application EP 0 501 387/B1.
One of the problems associated with prior art defrost controls, particularly those in which the defrost operation is initiated at regular time intervals, is that the defrost operation may be initiated at times when there is really not a need to defrost the evaporator(s). Further, even if there is a need for defrosting, there may be certain times of day (ie., periods of peak cooling requirements) during which it is not desirable to interrupt normal system operation in order to defrost the evaporator(s).
There is therefore a need for improved apparatus for controlling the defrost operation in a space cooling system.